Thermal stability of textile fibers



United States Patent 3,507,608 THERMAL STABILITY OF TEXTILE FIBERS Frederick R. Eirich, Eastchester, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Feb. 11, 1966, Ser. No. 527,384

Int. Cl. D06 U.S. Cl. 8--115.5 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to an improvement of the thermal stability of textile fibers wherein synthetic fibers are provided with a thin layer, coating or skin of an infusible polymer. Thereafter, although the synthetic fiber may melt, it will be retained by the polymer coating and become cross-linked therewith, reducing or avoiding the effects of melting.

All customary synthetic fibers, notwithstanding their various valuable properties, sulfer from disadvantages with respect to their thermal stability, especially thermal decomposition, melting and flammability. Elforts made towards improvements have not been very successful because of basic difliculties such as liability to be oxidized, limiting melting points, and various chemical degradations.

It is an object of this invention to provide synthetic fiber material, either filament or fabric, with an infusible sheath, coating or skin, i.e. a coherent coating of an infusible polymer in order to retard, reduce or prevent melting of the fibrous material when exposed to heat.

It has now been discovered that if synthetic fiber materials of the group consisting of linear polyamides and linear polyesters, either filaments or fabric, are coated with a thin layer of an infusible polymer, the melting synthetic fiber, either a nylon or polyester, can be contained within the infusible sheath long enough for itself to become crosslinked and thereby to reduce or prevent meltmg.

This discovery can find an explanation on the basis of a recently understood fact, namely that most polymers when exposed to heat will first degrade to lower molecular weights but after this will start to crosslink. If the latter of these two competing reactions is enhanced by an even shortlived containment as that within the infusible polymer sheath, melting can be greatly retarded or prevented and the fibers, fabric, or articles made thereof be stopped from dripping or flowing over adjacent objects. This is true for exposure of the synthetic and coated synthetic fibers to a mild flame or to the heat from a neighboring flame. As noted, under slower heating conditions, a reasonably good melt protection can be achieved by the method of infusible coating. An added advantage is that, if carried out as described under this invention below, the melt protection can be accomplished without appreciably impairing the textile properties of the original material.

The object of melt retardation of synthetic fibers, in particular of nylon type filaments or fabrics, can be achieved in yet another fashion which is related in mechanism with the one just described. During this embodiment of the invention, the nylon fiber is being superficially cross-linked by a chemical agent so that an infusible outer skin is formed which fulfills the same function as that of the infusible polymer sheath mentioned above. By this process, melting and dripping can again be prevented unless the material is being exposed to direct hot flames.

Extensive experimentation has shown that the desired object can be achieved while consideration is given to the desirable feature that any kind of melt-proofing sheaths should be resistant to the action of solvents or to launder- Ice ing. The way to accomplish this is by chemical anchoring of the melt proving agent to the surface of the material. It has been found that a method combining all the features consists in chemically activating the fabric surface by radiation which creates free radicals at or near the fiber surfaces, then exposing these radicals to the vapors of a monomer or monomers which polymerize to infusible polymers such as polyacrylonitrile, or divinylbenzene, and copolymers, and to allow the monomer vapors to grow into polymer from these anchoring points. If carried out as just described, or better of the coating will be found to be nonextractable. It has been found that coatings of a thickness which amounts to 12 to 20% of weight increase of the total filament is sufficient to give desirable melt retardation. That is, the material will not melt when heated on a hot metal block or when it becomes exposed to the flame of a match for several seconds.

The period of time during which the material has to be irradiated and exposed to the hot monomer vapor varies of course with intensity of radiation, the temperature of the material and vapor, and the concentration of the monomer vapor bath. Strangely enough, dipping of the exposed fabric into liquid monomer will not give the same good results. In general, 10 to 15 minutes exposure to monomer vapor and 1 to 2 minutes exposure to an ultraviolet source has found to be adequate. In addition both, exposure time to UV and vapor, can be shortened if the material has been previously supplied with a radiation sensitizer such as benzophenone, benzoin or many other UV absorbers. Benzoquinone is an example of another UV absorber. The latter is applied by a brief solution dipping whereby about an uptake of 0.1% or less of the sensitizer with respect to the weight of the fabric is sufficient. In this manner, the time of formation of the infusible sheath of, say, acrylonitrile can be reduced to onehalf of the time required without sensitizer, but the percentage of nonextractables is thereby reduced from over 75% to over 50%.

The second method of imparting melt retardation consists as stated in crosslinking a surface layer on the filaments. This can be carried out conveniently by the use of difunctional isocyanates by way of the biuret or allophanate reaction or, better, by diacidchlorides such as dichlorophosphate. Dichloroalkylphosphonates have been found particularly valuable. In this case, the chlorophosphorus compounds are mixed with a basic diluent and acid receptor, the nylon dipped into it, and the mixture refluxed. The fibers do not swell during this reaction, nor are there any unwanted side reactions. About 3% weight increase can be achieved within 10 to 30 minutes. The fibers are subsequently washed and found to be melt resistant to the exposure of mild flames or to heating. above the original melting point by adjacent flames. Textile properties remain good although the fibers obtain a slightly yellow tinge.

The compounds which were found satisfactory, as mentioned, are dichloroor dibromoalkylor arylphosphonic or phosphoric derivatives.

The following examples will illustrate the preferred embodiments of this invention.

EXAMPLE 1 A piece of nylon cloth of 3 x 3 feet is dipped into a bath of 5% benzophenone in ethanol for 5 minutes and then withdrawn and dried. It is then exposed to a battery of germicidal rod lamps of 3 feet length and 6 inches apart at a distance of 1 foot from the cloth for 3 minutes andthen is placed immediately on a fine wire netting which is suspended in cylindrical spiral form in a vertical tube which is fed with acrylonitrile vapors from boiling inhibited acrylonitrile for 10 minutes. The fabric is then withdrawn, cooled, briefly washed with water, drieqL and is ready for-use.

EXAMPLE 2 A piece'of nylon cloth of 3 x 3 feet is dipped into ;a bath of pyridine containing chlorornethyl phosphonic dichloride in rations of about 7:2 by volume. Any other high boiling base like lutidine or dimethylaniline can be used, or such other crosslinkers as phenylphosphonic oxydichloride. The material is held in the pyridine solution while it is refluxed at a temperature of 110-120 C. After this the material is removed, Washed with water. A 3% weight increase is noted after drying.

The mechanical properties and the feel of the samples as treated under Example 1 or 2 are very little changed, and the materials are now resistant to melting unless hit by a very hot flame.

I claim:

1. A process of making a synthetic linear polyamide fiber having individual filament provided with individual sheaths wherein the sheaths are cross-linked groups of the filaments consisting essentially of:

dipping said fiber into an ultraviolet absorbing solution; withdrawing said fiber from said solution and drying the treated fiber; irradiating said fiber with an ultraviolet source having one of the spectral lines of said absorbing solution for several minutes; immersing said fiber immediately into a bath of refluxing acrylonitrile vapor until a 10 percent to 20 percent weight increase is achieved; and

Washing and then drying said immersed fiber.

w 2, The proces s of claim 1 wherein said vapor bath contains additionally divinylbenzene.

3. The process of claim 2 wherein said ultraviolet absorber is either benzophenone or benzoin.

4. A process of making a synthetic fiber of the group consistingof' linear polyamide 'and linear polyesters having individual filaments provided .With' individual sheaths where the sheaths are grafted on the filaments consisting essentially of: 1

dipping said filaments in the clean state into a mixture of 2 parts by Weight of dichlorophosphate or chloromethyl phosphonic dichloride and 7 parts by Weight of pyridine;

refluxing from 10 to 30 minutes;

withdrawing said filament from said mixture; and

washing and then drying said filament;

5. A product produced by the process of claim 1.

References Cited UNITED STATES PATENTS 6/1968 Magat et a]. 8115.5,XR

MAYERWEINBLATT, Primary Examiner Us, or. X.R. 

